The present invention relates to artificial (e.g., man-made) quantum mechanical systems, and more specifically, to superconducting quantum circuits and devices suitable for operation at cryogenic temperatures.
Superconducting quantum circuits containing Josephson junctions are currently being pursued as the information-storing building blocks (i.e., quantum bits, or qubits) of a quantum computer. A basic challenge towards this goal is developing devices whose quantum coherence lasts long enough to enable control and measurement with error rates below the bounds requisite for quantum error correction.
Typical superconducting qubits are manufactured with aluminum thin films deposited on an insulating substrate of silicon or sapphire. A common known design, known in the literature as “circuit QED,” involves capacitively or inductively coupling the qubit circuit to an auxiliary high quality factor (Q) microwave-frequency resonator. This resonator can play multiple roles: it can filter the electromagnetic environment seen by the qubit mode; it can be energized with a signal at or near its resonant frequency in order to produce a measurement of the state of the qubit; or, in a multiple qubit device, it can facilitate coupling of one qubit to another.
The resonator may be formed, like the qubit, from thin films, and can have lumped element or transmission line segment geometry. The circuit QED system may also be based on a three-dimensional (3D) cavity, a resonant structure into which the entire chip with the qubit patterned upon it is placed. The primary distinguishing features of 2D versus 3D circuit QED devices involve the structure used to define the conducting or superconducting boundaries of the electromagnetic eigenmode used as the linear resonant mode. In a 2D circuit QED system the mode boundaries are formed by a predominantly planar structure patterned through the same or similar process as the qubit itself (though the physical fields of the resonator mode, as with the qubit mode, may have structure in three dimensions). In a 3D circuit QED system the boundaries have features and length scales in all three spatial dimensions of roughly comparable length scales. While a 2D circuit QED system contains a resonator patterned on a chip, which along with other elements of the device is enclosed in a conducting or superconducting enclosure, a 3D circuit QED system employs as the resonator an eigenmode of the enclosure itself. The 3D circuit QED system is therefore distinguished by the absence of a planar circuit patterned on a substrate that implements the resonant mode.
In known 3D circuit QED devices the resonator is superconducting. A superconducting resonator is able to attain much higher quality factors than a normal metal resonator. A superconducting resonator can also act as a magnetic shield for the qubit. However, when a superconducting device undergoes the transition to the superconducting state at cryogenic temperatures, the thermal conductivity of the cavity walls is suppressed by several orders of magnitude. It then becomes difficult to further cool the chip and qubit, leading to insufficient thermalization of the qubit to the desired operating temperature.
Regardless of the geometry or design, in order to operate the system as an information storing quantum bit one must be able to create and sustain an arbitrary superposition of the quantum circuit eigenstates encoding for logical ‘0’ and logical ‘1’. One requirement for this is that the available thermal energy be much less than the energy separation between states, kT<<hf, where h is Planck's constant, f is the transition frequency between the circuit eigenstates encoding 0 and 1, T is the temperature of the qubit environment, and k is the Boltzmann constant. In order to enter the superconducting state for aluminum-based devices this temperature needs to be at or below about 1.2K. However this temperature is not sufficient for operation as a reliable quantum circuit, as typical qubit transition frequencies are in the 4 to 10 GHz range, corresponding roughly to 0.2K to 0.5K.
For this reason, operation and measurement of superconducting qubit devices is usually performed at or below about 20 mK. The typical system used to attain this temperature is a dilution refrigerator, though other systems, such as an adiabatic demagnetization refrigerator, are common. Regardless of the specific of system, the refrigerator provides, at its lowest temperature stage, a thermal reservoir at the desired operating temperature. The qubit device is mechanically and thermally anchored to this thermal reservoir. Because the known 3D circuit QED devices are based on an aluminum cavity, the qubit chip itself may not be in good thermal contact with the reservoir due to the thermal impedance of the bulk superconductor. This can occur even though the device is mechanically connected to the reservoir at the lowest temperature stage of the refrigeration system, as the superconducting walls of the cavity place a thermal impedance between the qubit chip (which is interior to those walls) and the thermal reservoir at the desired operating temperature. In known devices it is therefore very difficult to attain proper thermalization of the qubit to the desired operating temperature.